The present invention relates to improved apparatus and methods for dissolving oxygen in water. More particularly, the present invention relates to dissolving oxygen in a large body of water.
In the rearing of fish, in impoundments for commercial and recreational purposes, it has been found that fish have environmental conditions which must be provided to insure their maximum growth and survival in impoundments. Water temperature and oxygen content are critical environmental conditions. Although each species of fish has different temperature requirements, it has been found that if the water becomes either too hot or too cold, the growth rate of the fish will be substantially reduced while the mortality rate is increased. In addition, if the oxygen content of the water is too low, a decrease in growth rate is experienced, and if the content falls below a set minimum, a high mortality rate will result. In addition to the direct effects of water temperature and oxygen content on fish, there are also indirect effects, in that, undesirable temperatures and oxygen content can materially reduce the production of the natural foods in the water.
There is a tendency for water in small impoundments and those located in hotter climates to thermally stratify. This stratification is characterized by the formation of a hypolimnion layer below a thermocline. In this situation, circulation and mixing of the water above and below the thermocline is effectively non-existent preventing the existence of a homogenous body of water in the impoundment. In fact, wide variations in conditions can exist between the water above and below the thermocline. Normally, in hot weather the water in the hypolimnion layer will have a lower temperature and oxygen content than the water above the thermocline. If left undisturbed over a period of time, the oxygen content of the water in the hypolimnion layer can be very low.
When stratification occurs, it is possible for all the water located above the thermocline to be above the optimum temperature for a given species and that only water in the hypolimnion layer to be at an acceptable temperature. In this situation, the fish will attempt to move into the cooler hypolimnion layer. As the fish attempt to move into the hypolimnion layer, they will be suspended in the low oxygen content water and will either perish or return to the hot water above the thermocline. Thus, the stratification of the lake can materially reduce the usable area of the lake, in that, the fish are confined to that area of the lake which is above the thermocline. In the commercial production of fish in small impoundments, this reduction in usable area can become quite important.
If stratification continues for a period of time, the hypolimnion layer can become oxygen starved to the point that insect larva growth is inhibited. In some cases, it is believed that insect larva consists of eighty percent of the natural food of fish. Therefore, the lack of larva growth in impoundments can substantially increase the amount of supplemental foods which must be supplied to the fish.
In addition, long periods of stratification produce a high biological oxygen demand in the hypolimnion layer. If the impoundment were to mix or "turn over" due to the presence of a cold rain or wind on the surface of the lake, the movement of this water with a high biological oxygen demand to the surface can be tragic in that a fish kill can result. Similar problems can be present in maintaining a fish population by controlling the oxygen content of water in a moving body of water due to such factors as waste oxidation and the like.
The necessity of providing a proper water temperature and oxygen content environment for fish populations has long been recognized. Numerous methods have been suggested to satisfy this need. In one method, destratification is accomplished by removing water from the hypolimnion layer and spraying it into the air for aeration. In another method, water from above the thermocline is pumped down and discharged at a point below the thermocline in the hypolimnion layer. In other methods, the hypolimnion layer is aerated directly. An example of such a system is shown in U.S. Pat. No. 3,643,403 wherein air is bubbled in the water of the hypolimnion layer. The air bubbles are caused to flow downward through a chamber and are released to flow in an upward direction.
Although the above described methods produce aeration of water, they can destroy the extremely valuable reserve of cold water of the hypolimnion layer through destratification of the impoundment.
The method and apparatus of the present invention is especially beneficial in dissolving oxygen at an efficient rate in the water in the hypolimnion layer without disturbing the stratification of the impoundment. In addition, the present invention provides a trapped volume of oxygen submerged in the impoundment which causes an efficient absorption of oxygen by the water through surface contact with the oxygen.
According to a preferred embodiment of the present invention, an apparatus is provided with an open bottom chamber submerged in an impoundment at or near the bottom preferably below the thermocline. Oxygen is supplied to the chamber at a rate equal to the absorption of the oxygen into the water so that gases are not allowed to flow in an upward direction to disturb the thermocline. In another embodiment, flow-inducing means are provided to cause water in the hypolimnion layer to flow past the area of surface contact and are agitated during said flow. Flow of the water is created in a horizontal direction to avoid disturbing the thermocline. In another embodiment, a plurality of these chambers can be vertically spaced in an assembly and submerged in a moving body of water. A plurality of these assemblies can be positioned along the flow path to cause repeated contact until sufficient aeration or oxidation results.